biological organisms are instead generated by processes whose basic rules are extremely simple—and are often chosen essentially at random.

The pictures below show some typical examples of patterns found on mollusc shells. Many of these patterns are quite simple. But some are highly complex. Yet looking at these patterns one notices a remarkable similarity to patterns that we have seen many times before in this book—generated by simple one-dimensional cellular automata.

Typical examples of pigmentation patterns on mollusc shells. In each close-up the pattern grows from top to bottom, just like in a one-dimensional cellular automaton. Patterns with triangles are often said to have a "tent" or "divaricate" form. The shell on the bottom right is a slightly rare specimen where something close to an explicit nested pattern can be seen. Most of the shells are between one and four inches long; the one on the bottom right is nine inches long. The patterns are all various shades of brown on roughly white backgrounds. The shells are the following types: first row: Elliot's volute, vexillate volute, lettered cone; second row: music volute, banded marble cone, tent olive; third row: bough cone, textile cone, false melon volute (Livonia mammilla).